Apparatus for transformer tap changing under load



June 17, 1941. A. PALME ETAL APPARATUS FOR TRANSFORMER TAP CHANGING UNDER LOAD Filed March 29, 1940 llllllllllllllllllllllllllllllllllllllllllllll I W u e n n m r if? w Mpc t r m/Mi. n 9 mm h r T Q Patented June 17, 1941 APPARATUS FOR TRANSFORMER TAP I CHANGING UNDER LOAD ai-nm' Palme and Orin r. McCarty, Pittsiield. Masa, assignors to General Electric Company, a corporation of New York Application March 29, 1940, Serial No. 326,733

7 Claims.

This invention relates to transformer tap changing apparatus and is in the nature of an improvement over that shown in- Patent No. 2,112,064, Bl'ume, which patent is assigned to'the General Electric Company, the assignee of the present application.

The tap changer disclosed in the above patent is particularly adaptable for changing the transformer taps under load in heavy duty circuits. The transformer current is initially divided between two branch conductors which are each connected to the same tap on one of the transformer windings. A selector switch is provided for connecting a power circuit to either one or both of the branch conductors. In a tap changing operation the selector switch is adjusted so that the power circuit is connected to the transformer through just one of the branch conductors, and the circuit through the second branch tank, which bushings because of the high voltages involved are large and expensive.

It is therefore an object of this invention to provide a new and improved tap changing arrangement in which arcing at the contacts of the selector switch is substantially eliminated so that this element of the apparatus may safely be arranged within the main oil filled transformer tank.

For a consideration of what we believe to be novel and our invention, attention is directed to'the following description and the claims appended thereto taken in connection with the accompanying drawing.

Referring now to the single figure of the accompanying drawing, we have shown therein diagrammatically an embodiment of our invention as applied to a power transformer I. As shown, this transformer has a primary windconductor is interrupted by an arcing contactor. ing 2 connected Supply circuit 3 and a The tap change is then made by the transformer ratio adjuster which connects the second branch conductor to the next tap. The contactor is then closed after which the selector switch is shifted to connect the power circuit to the second branch conductor. The arcing contactor is then opened again to open circuit the first branch conductor which is then connected by the ratio adjuster to the same tap as is connected to the second branch conductor. The arcing contactor is then reclosed and the selector switch is shifted to connect both branch conductors to the power circuit. In this manner connections are changed in a step-by-step manner along a row of transformer tap'contacts without at any time breaking the main circuit.

While the greater amount of arcing takes place at the arcing contactor, a certain amount of arcing also takes place at the'contacts of the selector switch, due to the reactive impedance drop of the loop circuit including the closed arcing contactor. Because of this fact it is necessary to arrange both the selector switch and the arcing contactor exteriorly of the transformer t'ank' proper. As is well understood, any arcing taking place within the oil filled transformer tank causes a contamination or sludging of the oil resulting in a serious impairment of its insulation characteristics. To mount both the selector switch and arcing contactor outside of the transformer tank necessitates the provision of a plurality of bushings in addition to, the regular number through the wall of the transformer ondary winding 4 connected to a load circuit indicated by conductors I and 6. The winding 4 is provided with a plurality of tap contacts I. A suitable twin ratio adjuster 8 of any well known construction is arranged to make selecplication. The branch conductors 8 and it are arranged for selective simultaneous or individual connection to the load circuit conductor I by means of a main current carrying selector switch ti arranged within the transformer tank. This switch is substantially a non-current interrupting disconnecting switch and is shown by way of example as consisting of a segmental contact member H which in the position shown bridges the fixed contacts I! and I! connected to the branch conductors 8 and I0, respectively, and an intermediate fixed contact is connected to the load circuit conductor 5. The contacts l3,' l4, and ii are fixed upon an insulating base plate l6 and the segmental contact member is pivotally mounted at I! thereon. The segmental contact member II is operated through stem I! by a cam l9.

The selector switch is mounted upon the left end of a bushing II which extends into the transformer tank. The right end of the bushing extending outside of the transformer tank supports an arcing contactor indicated generally at 22. A

boxlike compartment defined in part by upper and lower walls 23 and 24, respectively, and an end wall 25 is secured to the side wall 28 of the transformer tank enclosing the right end of the bushing 2| and the contactor 22. This compartment is sealed from the interior of the main transformer tank and is normally filled with oil which may be poured into the compartment through any suitably arranged inlet (not shown).

The contactor 22 may be of any suitable type and as shown comprises a pair of contacts 21 and 28 arranged for axial movement within a surrounding arclng chamber defined by a cylindrical member 29 of insulating material and metal end caps 30 and 3|. The upper contact 21 is carried by a stem 32 extending through cap 30 and is biased downwardly by spring 33. The lower contact 28 is carried by stem 34 extending through the lower cap 3|. The cylinder 29 is' provided with an opening 35 through which oil may flow into and out or the arcing chamber. An insulator 31 connects the stem 34 with a second stem portion 38 extending through bushing 39 in the wall 24. The contact 28 is actuated through stem 34-38 by a closed cam generally indicated at 45.

The contacts 21 and 28 of the arcing contactor are connected across the branch conductors by a series loop conductor arrangement having negliglble reactive impedance extending through the bushing 2|. The bushing is substantially symmetrical on opposite sides of the central metal flange ring '43 by which the bushing is secured to the periphery of the opening 44 through the tank'wall 26. The bushing further includes a pair of cylindrical insulator portions 45 and 45 with metal co1lars41 and 48 having axial openings therethrough arranged over the outer ends of the insulator portions 45 and 45, respectively. Smaller. cylindrical insulators 49 and 55 are suitably flanged for cooperatively fitting into the axial openings'in the collars 41 and 48, while the outer ends thereof are covered with metal cap pieces and 52, respectively. The bushing assembly described is clamped together by means of an axially extending conductor rod 54 which is threaded at its opposite end for cooperatively receiving the nuts 55 and 55. The conductor rod 54 extending axially through the bushing assembly is provided with the sleeve 51 of insulating material and around which is arranged a second conductor concentric with the conductor 54 which comprises a metal sleeve .58 having terminals 59 and 5| at the opposite ends thereof. The selector switch is attached to the left end of the bushing, the base l5 of the selector switch being suitably secured to the arms 52 and 53 integral with the bushing end cap 5| and also to an arm 64 extending from the bushing collar 41. The contactor 22 is mounted upon the right end of the bushing, the upper cap 30 of the arcing chamber being secured to the bushing end cap 52 through the integral arm '55 D while the lower cap 3| is secured to the bushing collar 48 through the arm 51.

The circuit through the bushing for the arcing contactor 22 may be traced from the branch conductor I0 through the left end cap of the bushing, through the central conductor rod 54, the flexible lead 58 to the stem 32 of the upper contact member 21. The branch conductor 9 is connected to the lower contact element 28 of the contactor 22 by a circuit including the bushing collar 41, lead 59, terminal 59, the outer concentric cylinder 58, the terminal 6|, lead 1|,

the bushing collar 48, arm 61, flexible lead 13 and stem 34 to the lower contact 28;

The ratio adjuster 8 and the cams l9 and 48 are coupled together mechanically so that they all may be driven by a common operating means; As shown, they are all coupled together by a shaft 15 driven by a motor 18. The cam I9 is engaged by a roller 18 arranged on the lower end oi the operating stem l8 for the selector switch H. An electrical insulator 19 is arranged in the operating stem l8, and the lower end of the stem is guided by a suitable bearing provided in a stationary part 8| which may be suitably secured to the transformer tank. The roller 18 is maintained in engagement with the cam surface by means of a spring 82 arranged between the stationary part 8| and a stop 83 provided on the stem. The cam 48 for operating the arcing contactor 22 is of a closed or grooved typein which the roller 84 secured to the lower end of the operating stem 38 rides betweenthe inner and outer surfaces. With this type of cam the contacts of the arcing contactor may be positively separated in the event they are welded together upon heavy arcing. It will be noted that the cam 40 is arranged exteriorly of the transformer tank, while the other cam |9 may be arranged inside of the tank, the shaft 15 passing through the tank wall at the bushing 81.

Connected in series with the branch conductors 9 and Ill are the halves of a preventive reactor. Such devices are well known and serve to limit the circulating current through the branch conductors at the times when they are connected to different voltage tap contacts. When the branch conductors are connected to the same tap contacts, the current in the two halves 8|! of the reactor neutralize each other electromagnetlcally and no current limiting reactance is set up in the circuit.

By way-of example wehave shown our invention as applied to an automatic voltage regulating system. The control element is a conventional contact making voltmeter 9| connected to respond to the voltage of the load circuit 5, 5. It has a set of raise" contacts 92 which close when the load circuit voltage is below a predetermined normal value and a set of lower" contacts 93 which close when the circuit voltage is above the predetermined value. These "raise and lower" contacts 92 and 93 control reversing circuits for the motor 18 which is shown by way of example as a conventional reversible capacitor motor, such as is commonly used for driving the tap changing mechanism of regulating transformers. Ordinarily, of course, auxiliary relays are interposed between the contact making voltmeter contacts and the motor circuit, but for the sake of simplicity they have been omitted from the drawing.

The operation of the illustrated embodiment of our invention is as follows:

Initially, all of the transformer load current divides equally between the branch conductors 9 and III, which as shown are both connected to the same tap contacts by the ratio adjuster-8. Substantially all of this current will flow through the switch H which is designed'to carry all of this current. Assume that for some reason the as viewed in the drawing. As the shaft 15 begins to rotate in a counterclockwise direction, the roller 19 will first drop from the surface 95 onto a lower surface 96 of cam 19 resulting in movement of the contact 12 of switch ll so that it bridges only the fixed contacts l3 and I5. The current in the branch conductor I is not interrupted but continues to flow through the loop circuit bridging the fixed contacts l3 and I4 and comprising the concentric conductors 54 and 59 of the bushing and the current interrupting contactor 22. Shortly after the contact H of the switch ll leaves the fixed contact H, the roller 84 will drop off of the surface 91 of the cam 40 onto the lower surface 98, thereby separating the contacts 2! and 29 of the arcing contactor 22. The current flow through the branch conductor I9 is thereby interrupted and all of the transformer current will now be carried through the branch conductor 9. The branch conductor Hi can then be connected to the next lowermost tap, as viewed in the drawing, by means of the ratio adjuster 8. Shortly contact I2 of the selector switch II so that it bridges the fixed contacts I and i5 and disengages the fixed contact 13. Soon after this shift of the selector switch II, the roller 84 will drop from the surface 99 onto the lower surface ")2 of the cam 40 to open the contactor 22 and interrupt the current flow through the branch conductor 9. The branch conductor 9 may now be connected to the next lowermost tap contact, with which the branch conductor in is connected, by operation of the ratio adjuster. After this tap change has been completed, the roller 84 will rise up onto the surface 91 of the cam 40 to reclose the arcing contactor 22, completing a current carrying circuit through the branch conductor 9 and fixed contact ll of the selector switch II to the common contact l5. Further rotation of the motor will result in the movement of the roller 18 onto the surface 95 of the cam l9 and movement of the segment |2 of the selector switch H to the position shown in which it connects both the fixed contacts I3 and H to the common contact IS.

The voltage of the load circuit will now have been raised by an amount corresponding to the voltage between the tap contacts of the transformer and if this is sufficient to restore the voltage to normal, the contact making voltmeter will break the motor circuit through its "raise" contact 92. If not, the motor 16 will continue rotating and the cams will progress through another cycle of operation as described to connect the branch conductors 9 and In to a still lower tap of the transformer winding until the voltage has been raised to the value at which the contact making voltmeter 9| balances.

If the voltage of the load circuit 5, 6 is too high, the lower contact 93 of the contact making voltmeter 9| will close, thereby reversing the direction of the motor 16. This will not reverse the sequence of operation of the selector switch H and the contactor 22 but merely reverses the direction of the tap changing by the ratio ad- Juster. Thus instead-of connectingthe branch conductors 9 and I 0 successively to lower tap contacts, the reversal of direction of rotation of the motor 16 causes theratio adjuster to make successive connections in a step-by-step manner along the upper tap contacts I, thus lowering the voltage of the load circuit 5, 6. It is to be understood, however, that before each tap change with respect to one of the branch conductors, the circuit through it is first broken, by the selector switch H and then the current in it is interrupted by the contactor 22 before the tap change is made. This operation continues until the voltage is restored to normal whereupon the contact making voltmeter 9i balances.

By the arrangement described no arcing will take place at the fixed contacts l3 and I l of the selector switch H during the operation thereof due to the fact that the loop circuit to the arcing contactor 22 bridging the fixed contacts possesses substantially no reactive impedance. The magnetic fields produced by current flowing through the concentric conductors 5 and 58 of the bushing 2| substantially neutralize each other so that the resultant field and hence the inductance thereof is zero. By making all current carrying parts of this loop circuit of very low resistance the total impedance of the loop circuit is reduced to a negligibly low value. Under these conditions there will be substantially no impedance drop in the closed loop circuit and arcing at the contacts of the selector switch H is substantially eliminated so that the selector switch may be safely mounted within the transformer tank proper. Since the arcing contactor 22 is arranged within its own individual oil filled compartment, the transformer oil will not deteriorate as a result of the switching operations.

Having described the principle of operation of our invention together with the apparatus which we now consider to represent the best embodiment thereof, we desire to have it understood that the apparatus shown is merely illustrative and that the invention may be carried out by other means.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, a power circuit, a transformer comprising a housing, a winding having a plurality of taps, a ratio adjuster in said housing for selectively connecting a pair of conductors to said taps, a non-current interrupting selector switch in said housing for selectively connecting either one or both of said conductors to said power circuit, a current interrupting contactor for interrupting the current in whichever conductor has been disconnected from said circuit by said selector switch, an enclosure for said contactor, an insulating bushing extending through a wall of said transformer housing into said enclosure, and a pair of concentrically arranged conductors extending through said bushing for connecting said contactor to said first pair of conductors.

2. In a circuit for transformer tap changing under load, a power circuit, a transformer winding provided with a plurality of taps, a ratio adjuster for selectively connecting a pair of conductors to said taps, main current carrying selector switching means for connecting both of said conductors to said power circuit and for selectively disconnecting either one of said conductors rom said circuit, a current interrupting a current 0011- arcing contactor for maintaining tric conductors connecting "gaging but one of circuit, a transformer ducting path between said circuit and the conductor disconnected therefrom by said switching means until after said switching means opens, and a loop circuit comprising a pair of concensaid arcing contactor to said first pair of conductors.

3. In a circuit for transformer tap changing under load, a transformer winding provided with a plurality of taps, a ratio adjuster, ior selectively connecting a pair of conductors to said taps, a switch having two fixed contacts and a movable contact, said fixed contacts being connected respectively to said conductors, said movable contact being arranged selectively to engage both or either or said fixed contacts, a contactor, means connecting said contactor across said fixed contacts, said switch and said contactor bein so correlated that said when the movable contact of the switch is enthe fixed contacts, said conbetween said switch and said conpair of conductors so arnecting means tactor comprising a contactor is opened only i tween said switch and ranged as to have negligible resultant reactance whereby substantially no arcing takes place at said switch when it is operated with said contactor closed.

4. In a circuit for transformer tap changing under load, the combination comprising a power an enclosing tank containing an insulating liquid, 9. switch in said tank for connecting a lead of said transformer winding to said circuit, an arcing contactor arranged outside of said tank and connected across contacts of said switch, said switch being adapted to be operated only when said arcing contactor is closed, the connection between said switch and said contactor comprising a loop conductor circuit having negligible reactance whereby substantially no arcing takes place at said switch when said switch is operated.

5. In combination, a tank containing an insulating liquid, a transformer in said tank having a winding provided with a plurality of taps,

including a winding and ..ing, an oil filled said anions:

a switch arranged in the liquid in said tank and having a pair of contacts normally connected to said taps, an arcing contactor arranged outside of said tank and connected across said contacts. said switch being adapted to be operated only when said contactor is closed, the connection besaid contactor comprising a loop conductor circuit having negligible resultant impedance.

6. In combination, 'a tank containing an insulating liquid, a transformer in said tank having a winding provided with a plurality of taps, a bushing arranged through a wall oi! said tank, a switch arranged on the end of said bushing on the interior of said tank and having a pair of contacts normally connected to said taps, an arcing contactor arranged on the end of said bushing outside of said tank, means for sequentially operating said switch and said contactor whereby said switch is operated only when said contactor is closed, said contactor being connected across switch contacts by a loop circuit including a pair of concentric conductors extending axially through said bushing.

'7. In combination, latlng liquid, a transformer a winding provided with a plurality of taps, a bushing extending through a wall or said tank. a switch arranged in said tank adjacent the inner end 01' said bushing, said switch having a pair of fixed contacts normally connected to said taps, an arcing contactor arranged outside of said tank adjacent the outer end or said bushing, a loop circuit connecting said contactor across said pair of fixed contacts comprising a pair'oi concentric conductors extending through said bushenclosure for said contactor, and means for operating said switch and said contactor whereby said switch may be operated only while said contactor is closed.=

ARTHUR PALME. 0am P McCARTY,

a tank containing an insu in said tank having 

